The invention relates to coating methods and, more particularly, to methods for coating fluid layers on a moving web to form magnetic recording media such as magnetic tape and diskettes.
Data storage media such as magnetic tape and diskettes typically are manufactured by coating one or more magnetic layers on a substrate, and then drying the resultant coating to form a film. For manufacturing reasons, the substrate ordinarily takes the form of a moving web that is transported relative to a generally fixed coating apparatus.
In an effort to store increased amounts of information, it is desirable to provide higher density magnetic recording media. Higher storage density can be achieved by including increased amounts of magnetic particles in the magnetic layer, adding additional magnetic layers, using thinner layers, or providing magnetic particles capable of increased data storage density.
The substrate may be provided with a subbing layer that is typically situated between the substrate and the magnetic recording layer. A subbing layer can promote adhesion between the substrate and the magnetic recording layer, whether the media contains one or more magnetic recording layers. Thus, a magnetic recording medium may contain a subbing layer and one or more magnetic recording layers thereon, resulting in a multi-layer construction.
Producing magnetic recording media with a multi-layer construction typically has involved sequential coating and drying steps, adding one layer during each coating application. Existing coating techniques include roll coating, gravure coating, extrusion coating, and a combination thereof, to name a few. These types of coating methods are generally insufficient when it is desirable to coat all layers in a single step in the production of a multi-layer construction.
Although multi-layer coating techniques exist, characteristics of the magnetic layer composition pose difficulties. In particular, the relatively high viscosity of typical magnetic coating compositions, as well as potential aggregation of the magnetic particles within the magnetic coating composition, can undermine the effectiveness of existing multi-layer coating techniques. Coating difficulties can become even more significant when the speed of the web is increased, the thickness of the magnetic layer is decreased, or both.
The invention is directed to magnetic recording media such as magnetic tape and diskettes. A coating method takes advantage of fluid coating formulations having a particular rheology that enables the coating of one or more magnetic layers with reduced thicknesses while achieving increased coating speeds. With proper rheology, one or more layers can be coated on a substrate traveling at increased speeds while maintaining desired coating thicknesses and quality.
The method may enable increased coating speeds by providing an enlarged coating window and reduced layer thicknesses that produce reduced drying loads. Notably, the method can be used to coat multiple layers of reduced thickness simultaneously, offering increased manufacturing throughput and improved magnetic recording performance.
In further embodiments, the method can produce multiple layers with both reduced thickness and improved coating uniformity, contributing to enhanced recording performance and increased recording density. Increased surface uniformity between coated layers, in particular, can provide reduced modulation and improved recording performance.
The reduced coating thicknesses also can reduce the so-called thickness loss that occurs as a function of increased thickness when short wave magnetic recording techniques are used. Thickness loss can be avoided even though dispersions of magnetic particles in a binder system are used, rather than vapor deposition, to achieve desired coating thicknesses.
In addition, the use of fluid coating techniques such as slide coating produces reduced loading force on the substrate, permitting the use of thinner substrates. Thinner substrates also contribute to a reduction in thickness loss. Further, in the case of magnetic tape, reduced layer and substrate thicknesses promote reduced tape thickness that may permit more tape for a given tape pack volume and, consequently, higher storage capacity.
A method in accordance with the invention preferably makes use of slide coating techniques for simultaneous delivery of multiple layers to the substrate, although other coating techniques may be useful. Slide coating is a method for multi-layer coating, i.e., it involves simultaneous coating of a plurality of fluid layers onto a substrate. The different fluids forming the multiple layers flow out of multiple slots that open out onto an inclined plane. The fluids flow down the plane, across the coating gap and onto an upward moving substrate. The fluids generally do not mix on the plane, across the coating gap, or on the web, so that the final coating is composed of distinct superposed layers.
Careful selection of coating formulation rheology, in accordance with the invention, provides a slide coating method with an enlarged coating window, i.e., a wider range of coating speeds and thicknesses. Viscosity in high shear rate regions, e.g., at the coating bead, determines coatability and defines maximum coating speed. Viscosity in intermediate shear rate regions, e.g., on the slide coater, governs layer thickness as the coating composition flows down the slide surface. Viscosity in low shear rate regions, e.g., following application of the coating on the substrate, determines mobility of the coating and resistance to defects in the drying zone.
Proper rheology of the coating formulation permits the coating of extremely thin films at high speeds with minimal coating defects in the finished magnetic media product. Generally, as speed increases, coating thickness must increase to stay within the coating window. As speed increases beyond a critical point, however, it has been found that coating thickness can actually be decreased with higher speeds. In particular, as speed increases, inertia that begins to dominate other forces, thereby permitting decreased coating thicknesses.
Reduced drying load due to reduced coating thicknesses in the coating system may enable higher web speeds and the use of smaller, less costly drying ovens. If desired, multiple coating layers can be applied in a single pass, permitting ready incorporation of different functional layers in the coated product, e.g., different layers for magnetic recording, head cleaning, antistatic, lubrication, and the like. To further improve coating performance, one or more saturation techniques may be used to prevent premature drying of highly evaporative solvents used to coat the substrate. As one example, a boundary layer saturator may be provided to produce a saturated environment adjacent the coating gap. The saturated environment reduces drying caused by the boundary layer of air carried into the coating gap by the moving substrate.
As another example, a slide coating face saturator can be provided to produce a saturated environment at the leading face of the coater, adjacent to the coating gap, to reduce drying of dispersion that accumulates on the face. In addition, a slide surface saturator can be provided to reduce drying of coating solution on the slide ramp surface. For each saturator, a saturated environment can be provided by introducing solvent vapor into the pertinent area.
The solvent vapor can be introduced using a variety of introduction devices including pressurized feeds, evaporation pans, and the like. In one embodiment, the solvent vapor may be introduced using a wicking or capillary material that uses capillary forces, i.e., surface tension, to draw solvent into the coating region and drive the solvent to the surface of the material. At the surface of the wicking material, the solvent evaporates into the coating region. The use of a capillary material may be advantageous as the introduction of solvent vapor is less disruptive to the surface of the coated substrate. In particular, whereas pressurized feeds may create currents that disrupt the surface of the coating, and create patterns that produce defects in the coated substrate, the evaporated solvent from a capillary material is less likely to cause defects.
In one embodiment, the invention provides a magnetic recording medium comprising a substrate, a first fluid-coated layer formed over the substrate, and a second fluid-coated layer formed over the first layer using a wet-on-wet multi-layer slide coating process, the second layer containing metal magnetic recording particles, wherein the first and second layers are coated with a combined average wet thickness on the substrate of less than approximately 25 micrometers.
In another embodiment, the invention provides a magnetic recording medium comprising a substrate, a first fluid-coated layer formed over the substrate, and a second fluid-coated layer formed over the first layer using a wet-on-wet multi-layer slide coating process, the second layer containing metal magnetic recording particles, wherein the second fluid-coated layer has a thickness on the substrate of less than approximately 3 micrometers.
In an added embodiment, the invention provides a magnetic recording medium comprising a substrate, a first fluid-coated layer formed over the substrate, and a second fluid-coated layer formed over the first layer using a wet-on-wet multi-layer slide coating process, the second layer containing metal magnetic recording particles, wherein the second fluid-coated layer has a thickness of less than approximately 0.4 micrometers and an average inter-layer roughness of less than approximately 10 nm.
In a further embodiment, the invention provides a magnetic recording medium comprising a substrate, a fluid-coated first layer formed over the substrate, a fluid-coated second layer formed over the first layer using a wet-on-wet multi-layer slide coating process, and a fluid-coated third layer formed over the second layer using a wet-on-wet multi-layer slide coating process, the third layer containing a magnetic recording material, wherein the first, second, and third layers have a combined thickness of less than approximately 3 micrometers.
In another embodiment, the invention provides a magnetic recording medium comprising a substrate having a thickness of less than 8 micrometers, a fluid-coated first layer formed over the substrate, a fluid-coated second layer formed over the first layer using a wet-on-wet multi-layer slide coating process, and a fluid-coated third layer formed over the second layer using a wet-on-wet multi-layer slide coating process, the third layer containing a magnetic recording material, wherein the first, second, and third layers have a combined thickness of less than approximately 3 micrometers.
The details of one or more embodiments of the invention are set forth in the description below. Other features, objects, and advantages of the invention will be apparent from the description, and from the claims.